Fix: updated RootConverter's unit tests

[jr-rk]

fix(tests): adjust RootConverters unit tests to assert CLARIN-DSpace instead of DSpace

Problem description

Backported #1405 was merged but unit 2 unit tests failed:

[ERROR] Failures: 
[ERROR]   RootConverterTest.testCorrectInternalUrlSetFromConfigurationService:87 expected:<[]DSpace 7.6.5> but was:<[CLARIN-]DSpace 7.6.5>
[ERROR]   RootConverterTest.testCorrectPropertiesSetFromConfigurationService:68 expected:<[]DSpace 7.6.5> but was:<[CLARIN-]DSpace 7.6.5>

Fix

Fixed testConfigurableVersionPrefix, which stubbed dspace.version.prefix with a stale default arg ("DSpace") left over from before the prod default changed to "CLARIN-DSpace", causing Mockito's strict-stub runner to flag it as an unused stub (UnnecessaryStubbingException) since the real code never called getProperty with that argument. Updated the stub's default arg to match and changed its return value to a distinct "CustomPrefix" so the test actually verifies override behavior instead of duplicating setUp(), and added testBlankVersionPrefixFallsBackToDefault to cover the blank-value fallback path.